1. Field of the Invention
The present invention relates to a lens array for converging the light from a document on a sensor (i.e., a light-receiving element) and an image-scanning device such as a copying machine, a facsimile machine, and a scanner in which the lens array is incorporated.
2. Description of the Prior Art
A contact-type image sensor in which an erecting 1:1 imaging lens (i.e., an erecting unit magnification lens) is incorporated can be made thin because the number of parts used is smaller than that of a reduced type image sensor. In this manner, the contact-type image sensor is used as an image-scanning device (i.e., an image sensor unit) such as a facsimile machine, a copying machine, and a hand scanner.
FIG. 9 is an entire structural view of a conventional contact-type image-scanning device shown in the following Patent Document 1. A casing 102 for housing a light guide 101 and a rod lens array 103 are disposed within a frame 100. A cover glass 104 is provided at the upper part of the frame 100 and a base plate 106 mounting a line image sensor (i.e., a light-receiving element) 105 thereon is provided at the lower part thereof. An illuminating light emitted from a light- emitting surface of the light guide 101 is caused to be incident on a document through the cover glass 104 and the reflected light is detected by the line image sensor 105 through the rod lens array 103, thereby scanning the document.
FIG. 10 is a perspective view of the rod lens array 103. The rod lens array 103 consists of a number of rod lenses 103a of which both ends are sandwiched between resin plates 103b. Black resin is filled between the rod lenses 103a. 
[Patent Document]
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-358906
The frame of the image sensor unit is generally a resin-molded article. Thus, the frame cannot be molded with a high degree of accuracy and its self-shape retention is also low. In this manner, it is difficult to install the rod lens array without inclination and as shown in FIG. 10, the displacement (Δy) is produced between the light axis of the rod lens array and the line image sensor.
FIG. 11 shows the result whereby the relationship between this displacement (Δy) and an amount of light level was verified along the longitudinal direction (i.e., a main scanning direction). It can be seen from FIG. 11 that the larger the displacement (Δy), the much lower the amount of light level.
In this manner, if deterioration of the amount of light level takes place, deformation in line is caused to lower the image quality when, for example, a linear image such as a ruled line is scanned.
To solve this problem, a rod lens array has been adopted in which the rod lenses are disposed in two rows in a sub-scanning direction. However, since the production cost becomes high even in rod lens array in one row, to have the rod lens array disposed in two rows makes the production yield worse to increase the cost. Further, even though the rod lenses are provided in two rows, it is not possible to completely eliminate the displacement of light axis.
On the other hand, if a micro lens array is employed, it is easy to have the lens disposed in many rows, but in the case of an ordinary micro lens array, it is not possible to provide an erect 1:1 imaging lens.
Further, in the case of the contact-type image-scanning device, there is also a problem of the deterioration in resolution resulting from the elevation of a document as well as the displacement between the light axis of the rod lens array and the line image sensor. In other words, when, for example, a book is mounted on a glass plate (i.e., a document table) in the open condition, a portion between pages is inevitably elevated.
In the conventional image-scanning device, since the irradiated light from an illuminating device is designed to be concentrated substantially on the glass plate (the document desk), the amount of light irradiated becomes uneven at a portion where the document is elevated to cause the resolution to deteriorate.
Further, even though the amount of light irradiated is dispersed to a certain degree in accordance with the elevation of the document, there is still a problem of focal depth. In other words, in order not to cause the resolution to deteriorate even though there is some elevation of the document, it is imperative to make the focal depth of the lens large.
FIG. 12 is a graph showing the relationship among an angle of aperture of the lens, the focal depth, and the amount of light transmitted. As is obvious from this graph, the focal depth and the amount of light transmitted are in a trade-off relationship. For example, the angle of aperture of the rod lens must be 6 degrees to correspond to a document elevation of 1 mm, but the amount of light transmitted in the case of the angle of aperture of 6 degrees becomes about 0.3%, which is very small. Thus, an insufficient amount of light results.